Co-Pyrolysis and Co-Combustion of Coal and Biomass

Abstract

Sustainability, security of supply, and diversity, as well as economic competitiveness are key components of energy policy. There is increasingly stringent legislation on the environmental impact of energy production, and there is growing pressure to reduce not just NOx and SOx emissions, but also C02 emissions. For both heating and electricity production it is likely that the plants will need to be fuel-flexible and could use one or more of several different feedstocks, for example coal and biomass. When coal is co-utilized with biomass there is added attractiveness because the biomass is C02 neutral, and there is interest in using wood waste, short rotation
woody crops (e. g. willow coppice), or herbaceous crops (e. g. Miscanthus), refuse and waste derived fuels, or wastes such as sewage sludge or chicken litter.
The co-utilisation of coal and biomass for heat and/or energy production results in pollutant reduction. Most notable is the impact on the emission of NOx, SOx, volatile
organic compounds and polyaromatic hydrocarbons. These latter compounds largely arise from their formation and release during incomplete combustion/gasification. There
is evidence that co-firing or co-gasifying coal and biomass results in a significant decrease in the emission of these
compared to coal alone.
The synergistic activity observed for toxic organic emissions is not well understood and is thought to involve chemical interaction between the volatiles from each fuel coupled with possible catalytic activity from the inorganic constituents of the fuels. Laboratory scale data on synergies in co-pyrolysis is conflicting. Characterisation
of co-pyrolysis products from coal and biomass pyrolysis has received limited attention and the data is conflicting. Therefore this thesis seeks to understand possible
interactions occurring during co-combustion and co-pyrolysis of fuels and looks at a number of variables, including coal rank, biomass type (with different amounts of
catalytic components), heating rate, residence time and the physical form of the fuels.
A better understanding of the factors influencing non-additive interactions may lead to optimization of the blending process and minimisation of toxic organic
emissions. This work is of particular relevance to fixed bed and fluidised bed processes where the bed temperature is ca. 1000 'C (or there is a temperature profile through the bed). In these cases particle heating and pyrolysis occurs relatively slowly and interactions between the volatiles can take place.
While studying the co-pyrolysis, thermogravimetry, batch pyrolysis and pyroprobe-GC/(MS or FID) were used. In addition, apart from the traditionaltechniques, this study aimed to develop a new technique - heated wire mesh pyrolysis coupled to a GUMS via a probe, which can sample at varying heights from the pyrolysing fuel, and these findings were complemented by the pyrolysis-GC/MS studies
of the fuels. These studies suggest that biomass type can lead to a small change of the rate of the coal pyrolysis. Thus, slight synergistic effects were seen for the TGA study, where co-pyrolysed coals in blends often had lower peak temperatures compared to the coal alone, and higher volatile matter yields were produced. Analysis of the gases
evolved were consistent with higher gas yields. This effect was present for certain biomass (e. g. oat straw) even after minerals were removed, and so this is not purely the
result of catalytic ash components.
For combustion studies two techniques were applied. Low heating rate was obtained in a TGA analyser. The high heating rate experiments were performed on pellets exposed to the flame of Meker-type burner. This combustion process was recorded with a high speed frame video recording system. These studies showed that strong synergy can be observed. The TGA combustion revealed the importance of the
catalytic elements, particularly potassium, and showed that, ignition of biomass char in the blend aids the ignition of the coal char. As a result, mixtures reach maximum temperatures faster, than seen for the separate fuels. In many cases though, the char burn-out of the blends lasted a similar time to the coals alone. The combustion tests of stationary pellets revealed no pattern for the ignition delay, but exposed strong synergy
in volatile combustion, indicating that for pellets of untreated fuel blends the combustion events are dominated by the coal behaviour i. e. the addition of demineralised biomass to the pellet, made it burn in a very similar way to coal alone.
The synergy observed in the organic emissions during the combustion of coal and biomass in small appliances is not simply due to interactions of hot volatiles from coal
and biomass above the combustion bed. Co-pyrolysis studies suggest that biomass type can lead to a small effect on the rate of the coal pyrolysis, and on the total volatile
matter released, but that there are no major changes in the nature of the volatiles. Combustion studies indicate that synergy stronger than seen for pyrolysis tests can be
observed, and the coal ignites and burns at lower temperature as a result of the earlier ignition and combustion of the biomass. The overall combustion time is still dominated by the coal char burn-out. Thus, synergy in emission reduction in the co-utilisation of coal and biomass is not simply due to interactions of volatiles in the vapour phase, rather, the processes of pyrolysis and combustion are linked and as such need to be studied together.